Memory devices and methods of manufacturing thereof

ABSTRACT

A memory device includes first nanostructures stacked on top of one another; first gate stacks, where two adjacent ones of the first gate stacks wrap around a corresponding first nanostructure; second nanostructures stacked on top of one another; second gate stacks, where two adjacent ones of the second gate stacks wrap around a corresponding second nanostructure; a first drain/source feature electrically coupled to a first end of the first nanostructures; a second drain/source feature electrically coupled to both of a second end of the first nanostructures and a first end of the second nanostructures; and a third drain/source feature electrically coupled to a second end of the second nanostructures. At least one of the plurality of first gate stacks is in direct contact with at least one of the first drain/source feature or the second drain/source feature.

REFERENCE TO RELATED APPLICATION

This application is related to and claims priority under 35 U.S. § 120as a divisional application of U.S. Utility application Ser. No.16/881,777, filed May 22, 2020, titled “MEMORY DEVICES AND METHODS OFMANUFACTURING THEREOF,” the entire contents of which are incorporatedherein by reference for all purposes.

BACKGROUND

Integrated circuits (ICs) sometimes include one-time-programmable (OTP)memories to provide non-volatile memory (NVM) in which data are not lostwhen the IC is powered off. One type of the OTP devices includesanti-fuse memories. The anti-fuse memories include a number of anti-fusememory cells (or bit cells), whose terminals are disconnected beforeprogramming, and are shorted (e.g., connected) after the programming.The anti-fuse memories may be based on metal-oxide-semiconductor (MOS)technology. For example, an anti-fuse memory cell may include aprogramming MOS transistor (or MOS capacitor) and at least one readingMOS transistor. A gate dielectric of the programming MOS transistor maybe broken down to cause the gate and the source or drain region of theprogramming MOS transistor to be interconnected. Depending on whetherthe gate dielectric of the programming MOS transistor is broken down,different data bits can be presented by the anti-fuse memory cellthrough reading a resultant current flowing through the programming MOStransistor and reading MOS transistor. The anti-fuse memories have theadvantageous features of reverse-engineering proofing, since theprogramming states of the anti-fuse cells cannot be determined throughreverse engineering.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates an example circuit diagram of a memory cell, inaccordance with some embodiments.

FIG. 2 illustrates a flow chart of an example method to fabricate amemory device, in accordance with some embodiments.

FIGS. 3-1, 3-2, 3-3, 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, 3-11, 3-12, and3-13 illustrate cross-sectional views of a memory device, made by themethod of FIG. 2, at various fabrication stages, in accordance with someembodiments.

FIG. 4 illustrates a cross-sectional view of another example memorydevice 400, in accordance with some embodiments.

FIGS. 5, 6, and 7 illustrate various example nanostructure transistorsincluding partial inner spacers, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In contemporary semiconductor device fabrication processes, a largenumber of semiconductor devices, such as silicon channel n-type fieldeffect transistors (nFETs) and silicon germanium channel p-type fieldeffect transistors (pFETs), are fabricated on a single wafer. Non-planartransistor device architectures, such as fin-based transistors, canprovide increased device density and increased performance over planartransistors. Some advanced non-planar transistor device architectures,such as nanostructure (e.g., nanosheet, nanowire ormulti-bridge-channel) transistors, can further increase the performanceover fin-based transistors partially due to the characteristic of itsconduction channel wrapped around by the respective gate feature.

Such a nanosheet transistor includes multiple semiconductor sheets. Themultiple semiconductor sheets may collectively form a conduction channelfor the nanosheet transistor. Each of the semiconductor sheets isseparated by a gate stack including a layer of electrically conductivegate and a gate dielectric. The gate stacks wrap around all sides of thesemiconductor sheets, thereby forming a gate-all-around (GAA) structure.Epitaxial growths on the ends of the semiconductor nanosheets formsource/drain features for the nanosheet transistors. Spacers can beformed between the gate stacks from the source/drain features of thenanosheet transistors. Such spacers are typically referred to as “innerspacers.” The inner spacers, formed of dielectric materials, canelectrically isolate the gate stacks from the respective source/drainfeatures, which may reduce parasitic capacitances induced therebetween(e.g., C_(gd), C_(gs)).

By adopting such a nanosheet transistor configuration to form thereading transistor of a memory cell (e.g., an anti-fuse memory cell),performance (e.g., switching speed) of the reading transistor can beimproved. However, the inner spacers, coupled between the gate stacksand source/drain features, may increase a time and voltage (typicallyreferred to as “T_(BD)” and “V_(BD),” respectively) to break down thegate dielectric. This is partially due to the presence of the innerspacers may decrease the contact area between the gate stacks andnanosheets, which in turn decreases the contact area of each of the gatestacks to the source/drain features. As such, overall performance (e.g.,operation speed, programming yield, etc.) of the anti-fuse memory cellmay be disadvantageously affected.

The present disclosure provides various embodiments of a memory cell ina nanosheet transistor configuration. In some embodiments, the disclosedmemory cell includes an anti-fuse memory cell constituted by aprogramming transistor and one or more reading transistors. Each of theprogramming transistor and reading transistor(s) includes a nanosheettransistor. The programming transistor may have less dielectricmaterials coupled between respective gate stacks and source/drainfeatures, and the reading transistor may have more dielectric materialscoupled between respective gate stacks and source/drain features. Forexample, the programming transistor of the disclosed memory cell caninclude one or more gate stacks that are in direct contact with at leastone of its respective source/drain features, while the readingtransistor can includes one or more gate stacks that are electricallyisolated from its respective source/drain features by inner spacers.

FIG. 1 illustrates an example circuit diagram of a memory cell 100, inaccordance with some embodiments. As shown, the memory cell (orsometimes referred to as a memory bit cell, a memory bit, or a bit) 100includes a first transistor 110 and a second transistor 120. Each of thefirst and second transistors, 110 and 120, may include an n-typemetal-oxide-semiconductor field-effect-transistor (MOSFET). Thetransistors 110 and 120 may each include another type of the MOSFET,e.g., a p-type MOSFET. In some other embodiments, at least one of thetransistors 110 or 120 may be replaced by another type of electronicdevices, e.g., a MOS capacitor, while remaining within the scope of thepresent disclosure. The first transistor 110 and the second transistor120 are electrically coupled to each other in series. For example,source of the first transistor, 1105, is connected to drain of thesecond transistor, 120D.

The memory cell 100 may be configured as a one-time-programmable (OTP)memory cell such as, for example, an anti-fuse cell. It is understoodthat the memory cell 100 may be configured as any type of the memorycell that includes two transistors electrically coupled to each other inseries (e.g., a NOR-type non-volatile memory cell, a dynamicrandom-access memory (DRAM) cell, a two-transistor static random-accessmemory (SRMA) cell, etc.).

When the memory cell 100 is configured as an anti-fuse cell, the firsttransistor 110 can function as a programming transistor and the secondtransistor 120 can function as a reading transistor. As such, drain ofthe first transistor 110D is floating (e.g., coupled to nothing), andgate of the first transistor 110G is coupled to a programming word line(WLP) 130; and gate of the second transistor 120G is coupled to areading word line (WLR) 132, and source of the second transistor 120S iscoupled to a bit line (BL) 134.

To program the memory cell 100, the reading transistor 120 is turned onby supplying a high voltage (e.g., a positive voltage corresponding to alogic high state) to the gate 120G via the WLR 132. Prior to,concurrently with or subsequently to the reading transistor 120 beingturned on, a sufficiently high voltage (e.g., a breakdown voltage (VBD))is applied to the WLP 130, and a low voltage (e.g., a positive voltagecorresponding to a logic low state) is applied to the BL 134. The lowvoltage (applied on the BL 134) can be passed to the source 1105 suchthat VBD will be created across the source 1105 and the gate 110G tocause a breakdown of a portion of a gate dielectric (e.g., the portionbetween the source 1105 and the gate 110G) of the programming transistor110. After the breakdown of the gate dielectric of the programmingtransistor 110, a behavior of the portion interconnecting the gate 110Gand source 1105 is equivalently resistive. For example, such a portionmay function as a resistor 136. Before the programming (before the gatedielectric of the programming transistor 110 is broken down), noconduction path exists between the BL 134 and the WLP 130, when thereading transistor 120 is turned on; and after the programming, aconduction path exists between the BL 134 and the WLP 130 (e.g., via theresistor 136), when the reading transistor 120 is turned on.

To read the memory cell 100, similarly to the programming, the readingtransistor 120 is turned on and the BL 134 is coupled to a voltagecorresponding to the logic low state. In response, a positive voltage isapplied to the gate of the programming transistor 110G. As discussedabove, if the gate dielectric of the programming transistor 110 is notbroken down, no conduction path exists between the BL 134 and the WLP130. Thus, a relatively low current conducts from the WLP 130, throughthe transistors 110 and 120, and to the BL 134. If the gate dielectricof the programming transistor 110 is broken down, a conduction pathexists between the BL 134 and the WLP 130. Thus, a relatively highcurrent conducts from the WLP 130, through the transistor 110 (nowequivalent to the resistor 136) and transistor 120, and to the BL 134.Such a low current and high current may sometimes be referred to asI_(off) and I_(on) of the memory cell 110, respectively. A circuitcomponent (e.g., a sensing amplifier), coupled to the BL 134 candifferentiate I_(off) from I_(on) (or vice versa), and thus determinewhether the memory cell 100 presents a logic high (“1”) or a logic low(“0”). For example, when I_(on) is read, the memory cell 100 may present1; and when I_(off) is read, the memory cell 100 may present 0.

FIG. 2 illustrate a flowchart of a method 200 to form a memory device,according to one or more embodiments of the present disclosure. Themethod 200 may be used to form an anti-fuse memory cell, including aprogramming transistor and a reading transistor, coupled in series. Itis noted that the method 200 is merely an example and is not intended tolimit the present disclosure. Accordingly, it is understood thatadditional operations may be provided before, during, and after themethod 200 of FIG. 2, and that some other operations may only be brieflydescribed herein.

The operations of the method 200 may be associated with cross-sectionalviews of a memory device at respective fabrication stages as shown inFIGS. 3-1, 3-2, 3-3, 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, 3-11, 3-12, and3-13. In some embodiments, the memory device may be included in orotherwise coupled to a microprocessor, another memory device, and/orother integrated circuit (IC). Also, FIGS. 3-1 to 3-13 are simplifiedfor a better understanding of the concepts of the present disclosure.Although the figures illustrate the memory device, it is understood theIC may include a number of other devices such as inductors, resistor,capacitors, transistors, etc., which are not shown in FIGS. 3-1 to 3-13,for purposes of clarity of illustration.

Referring first to FIG. 2, in brief overview, the method 200 starts withoperation 202 in which a substrate is provided. The method 200 proceedsto operation 204 in which an alternating series of first nanostructuresand second nanostructures are formed. The method 200 proceeds tooperation 206 in which a number of dummy gate stacks are formed. Themethod 200 proceeds to operation 208 in which a firstalternating-nanostructure column and a second alternating-nanostructurecolumn are defined. The method 200 proceeds to operation 210 in whichthe first alternating-nanostructure column is covered. The method 200proceeds to operation 212 in which respective end portions of the firstnanostructures of the second alternating-nanostructure column areremoved. The method 200 proceeds to operation 214 in which inner spacersare formed in the second alternating-nanostructure column. The method200 proceeds to operation 216 in which source features and drainfeatures are formed. The method 200 proceeds to operation 218 in whichan inter-layer dielectric is deposited. The method 200 proceeds tooperation 220 in which the dummy gate stacks are removed. The method 200proceeds to operation 222 in which the first nanostructures of the firstand second alternating-nanostructure columns are removed. The method 200proceeds to operation 224 in which gate dielectrics are deposited. Themethod 200 proceeds to operation 226 in which gate metal are deposited.

Corresponding to operation 202, FIG. 3-1 is a cross-sectional view of amemory device 300, cut by a plane perpendicular to the Y direction,which includes the substrate 302, at one of the various stages offabrication. The substrate 302 includes a semiconductor materialsubstrate, for example, silicon. Alternatively, the substrate 302 mayinclude another elementary semiconductor material such as, for example,germanium. The substrate 302 may also include a compound semiconductorsuch as silicon carbide, gallium arsenic, indium arsenide, and indiumphosphide. The substrate 302 may include an alloy semiconductor such assilicon germanium, silicon germanium carbide, gallium arsenic phosphide,and gallium indium phosphide. In one embodiment, the substrate 302includes an epitaxial layer. For example, the substrate may have anepitaxial layer overlying a bulk semiconductor. Furthermore, thesubstrate 302 may include a semiconductor-on-insulator (SOI) structure.For example, the substrate 302 may include a buried oxide (BOX) layerformed by a process such as separation by implanted oxygen (SIMOX) orother suitable technique, such as wafer bonding and grinding.

Corresponding to operation 204, FIG. 3-2 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,which includes an alternating series of first semiconductor layers 304,308, and 312 and second semiconductor layers 306, 310, and 314, at oneof the various stages of fabrication. The first semiconductor layers304, 308, and 312 may include SiGe nanostructures (hereinafter “SiGenanostructures 304, 308, and 312”), and the second semiconductor layers306, 310, and 314 may include Si nanostructures (hereinafter “Sinanostructures 306, 310, and 314”). In some embodiments, each of theSiGe nanostructures 304, 308, and 312 may include a SiGe layer with athickness in the nano-range (e.g., a SiGe nanosheet); and each of the Sinanostructures 306, 310, and 314 may include a Si layer with a thicknessin the nano-range (e.g., a Si nanosheet). It is understood the thicknessof the SiGe nanostructures 304, 308, and 312 and the Si nanostructures306, 310, and 314 may be reduced to a sub-nano-range (e.g., angstroms),while remaining within the scope of the present disclosure. Thealternating series of SiGe nanostructures 304, 308, and 312, and the Sinanostructures 306, 310, and 314 may be formed as a stack over thesubstrate 302, wherein the nanostructures 304-314 are disposed on top ofone another along a vertical direction (e.g., the Z direction). Such astack may sometimes be referred to as a superlattice. In a non-limitingexample, a percentage of Ge in each of the SiGe nanostructures 304, 308,and 312 ranges from 10% to 40%. It is understood the percentage of Ge ineach of the SiGe nanostructures 304, 308, and 312 can be any valuebetween 0 and 100 (excluding 0 and 100), while remaining within thescope of the present disclosure.

The alternating series of nanostructures can be formed by epitaxiallygrowing one layer and then the next until the desired number and desiredthicknesses of the nanostructures are achieved. Epitaxial materials canbe grown from gaseous or liquid precursors. Epitaxial materials can begrown using vapor-phase epitaxy (VPE), molecular-beam epitaxy (MBE),liquid-phase epitaxy (LPE), or other suitable process. For epitaxialsilicon, silicon germanium, and/or carbon doped silicon (Si:C) siliconcan be doped during deposition (in-situ doped) by adding dopants, n-typedopants (e.g., phosphorus or arsenic) or p-type dopants (e.g., boron orgallium), depending on the type of transistor.

Corresponding to operation 206, FIG. 3-3 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes a first dummy gate stack 316 a and second dummy gate stack316 b, at one of the various stages of fabrication. Each of the dummygate stacks, 316 a-b, includes a dummy gate and a hard mask. For examplein FIG. 3-3, the first dummy gate stack 316 a includes a dummy gate 318a formed over the Si nanostructure 314, and a hard mask 320 a formedover the dummy gate 318 a; and the second dummy gate stack 316 bincludes a dummy gate 318 b formed over the Si nanostructure 314, and ahard mask 320 b formed over the dummy gate 318 b.

In some embodiments, the dummy gate stacks 316 a-b may correspond toregions where the gate features of a programming transistor and readingtransistor of the memory device 300 will be formed. Although each of thedummy gate stacks 316 a-b is shown as a two-dimensional structure inFIG. 3-3, it is appreciated that the dummy gate stacks 316 a-b are eachformed as a three-dimensional structure to straddle the alternatingseries of first nanostructures 304, 308, and 312 and secondnanostructures 306, 310, and 314. For example, each of the dummy gatestacks 316 a-b may be formed over and around sidewalls of the firstnanostructures 304, 308, and 312 and second nanostructures 306, 310, and314. The dummy gates 318 a-b can be formed by depositing amorphoussilicon (a-Si) over and around the alternating series of firstnanostructures 304, 308, and 312 and second nanostructures 306, 310, and314. The a-Si is then planarized to a desired level. A hard mask (notshown) is deposited over the planarized a-Si and patterned to form thehard masks 320 a-b. The hard masks 320 a-b can be formed from a nitrideor an oxide layer. An etching process (e.g., a reactive-ion etching(RIE) process) is applied to the a-Si to form the dummy gate stacks 316a-b.

After forming the dummy gate stacks 316 a-b, gate spacers 322 a and 322b may be formed to extend along respective sidewalls of the dummy gatestacks 316 a and 316 b, as illustrated in FIG. 3-3. The gate spacers 322a-b can be formed using a spacer pull down formation process. The gatespacers 322 a-b can also be formed by a conformal deposition of adielectric material (e.g., silicon oxide, silicon nitride, siliconoxynitride, SiBCN, SiOCN, SiOC, or any suitable combination of thosematerials) followed by a directional etch (e.g., RIE). Such gate spacersmay sometimes be referred to as outer spacers. In some embodiments, thedummy gate stack 316 a, together with the gate spacers 322 a, may extendalong a horizontal direction (e.g., the X direction) by a firstdistance, D₁; and the dummy gate stack 316 b, together with the gatespacers 322 b, may extend along the X direction by a second distance,D₂, as shown in FIG. 3-3. The first and second distances, D₁ and D₂, maybe identical to or different from each other.

Corresponding to operation 208, FIG. 3-4 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,which includes alternating-nanostructure columns 324 a and 324 b, at oneof the various stages of fabrication. Subsequently to forming the gatespacers 322 a-b, the alternating-nanostructure columns 324 a and 324 bmay be formed by at least some of the following processes: using thegate spacers 322 a-b, the dummy gates 318 a-b, and the hard masks 320a-b as a mask to define the footprint of the alternating-nanostructurecolumns 324 a and 324 b, and etching the alternating series of firstnanostructures 304, 308, and 312 and second nanostructures 306, 310, and314 (shown in FIG. 3-3) to form the alternating-nanostructure columns324 a and 324 b. As such, each of the alternating-nanostructure columns324 a and 324 b includes a stack of alternating etched SiGe/Sinanostructures. For example, the alternating-nanostructure column 324 aincludes a stack of alternating etched SiGe nanostructure 325 a, etchedSi nanostructure 326 a, etched SiGe nanostructure 327 a, etched Sinanostructure 328 a, etched SiGe nanostructure 329 a, and etched Sinanostructure 330 a; and the alternating-nanostructure column 324 bincludes a stack of alternating etched SiGe nanostructure 325 b, etchedSi nanostructure 326 b, etched SiGe nanostructure 327 b, etched Sinanostructure 328 b, etched SiGe nanostructure 329 b, and etched Sinanostructure 330 b.

In some embodiments, each of the etched Si and SiGe nanostructures ofthe alternating-nanostructure columns 324 a may follow the horizontaldimension of the dummy gate stack 316 a and gate spacers 322 a; and eachof the etched Si and SiGe nanostructures of thealternating-nanostructure columns 324 b may follow the horizontaldimension of the dummy gate stack 316 b and gate spacers 322 b.Accordingly, each of the etched Si and SiGe nanostructures of thealternating-nanostructure columns 324 a may extend along the X directionby D₁; and each of the etched Si and SiGe nanostructures of thealternating-nanostructure columns 324 b may extend along the X directionby D₂.

Corresponding to operation 210, FIG. 3-5 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes a blocking mask 334, at one of the various stages offabrication. In some embodiments, the blocking mask 334 is formed tooverlay the alternating-nanostructure columns 324 a while remaining thealternating-nanostructure columns 324 b exposed. The blocking mask 331may be formed to have a sufficiently great thickness (or height) suchthat respective sidewalls of each of the etched SiGe nanostructures 325a, 327 a, and 329 a are fully covered. Formation of the blocking mask334 may allow one or more processes, which shall be discussed below, tobe performed on the alternating-nanostructure columns 324 b. Theblocking mask 334 may be formed of a material relatively resistant toetchants that can etch SiGe such as, for example, silicon oxide, siliconnitride, silicon oxynitride, SiBCN, SiOCN, SiOC, or any suitablecombination of those materials.

Corresponding to operation 212, FIG. 3-6 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,in which respective end portions of each of the etched SiGenanostructure 325 b, etched SiGe nanostructure 327 b, and etched SiGenanostructure 329 b (shown in FIG. 3-5) are removed, at one of thevarious stages of fabrication. During the removal of the respective endportions of the etched SiGe nanostructure 325 b, etched SiGenanostructure 327 b, and etched SiGe nanostructure 329 b, thealternating-nanostructure columns 324 a is covered by the blocking mask334. As such, SiGe nanostructures 335 b, 337 b, and 339 b can be formedto extend along the X direction by a distance less than D₂ (e.g., D₃),while the SiGe nanostructures 325 a, 327 a, and 329 a of thealternating-nanostructure columns 324 a still extend along the Xdirection by D₁. The SiGe nanostructures 325 a, 327 a, 329 a, 335 b, 337b, and 339 b may be later replaced by a number of gate stacks. Thus, theSiGe nanostructures 325 a, 327 a, 329 a may be herein referred to asSiGe sacrificial nanostructures 325 a, 327 a, 329 a for thealternating-nanostructure columns 324 a, and the SiGe nanostructures 335b, 337 b, and 339 b may be herein referred to as SiGe sacrificialnanostructures 335 b, 337 b, 339 b for the alternating-nanostructurecolumns 324 b.

In some embodiments of present disclosure, the end portions of theetched SiGe nanostructures 325 b, 327 b, and 329 b can be removed usinga first application, so called a “pull-back” process to pull the etchedSiGe nanostructures 325 b, 327 b, and 329 b back an initial pull-backdistance such that the ends of the SiGe sacrificial nanostructures 335b, 337 b, and 339 b terminate underneath (e.g., aligned with) the gatespacers 322 b. Although in the illustrated embodiment of FIG. 3-6, theends of each of the SiGe sacrificial nanostructures 335 b, 337 b, and339 b are approximately aligned with the sidewalls of the spacer 322 b,it is understood that the pull-back distance (i.e., the extent to whicheach of the SiGe sacrificial nanostructures 335 b, 337 b, and 339 b isetched, or pulled-back) can be arbitrarily increased or decreased. Thepull-back process may include a hydrogen chloride (HCl) gas isotropicetch process, which etches SiGe without attacking Si.

Corresponding to operation 214, FIG. 3-7 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes inner spacers 340, 342, and 344, at one of the variousstages of fabrication. During the formation of the inner spacers340-344, the alternating-nanostructure columns 324 a is covered by theblocking mask 334. As such, only the alternating-nanostructure columns324 a has inner spacers 340-344 disposed along respective sidewalls ofthe SiGe sacrificial nanostructures 335 b, 337 b, and 339 b. In someembodiments, the inner spacers 340-344 can be formed conformally bychemical vapor deposition (CVD), or by monolayer doping (MLD) of nitridefollowed by spacer RIE. In some other embodiments, the inner spacers340-344 can be deposited using, e.g., a conformal deposition process andsubsequent isotropic or anisotropic etch back to remove excess spacermaterial on vertical sidewalls of the alternating-nanostructure column324 b and on a surface of the semiconductor substrate 302. Accordingly,the inner spacers 340-344 may extend along the X direction by a distanceD₄, that is about one half of the difference between D₂ and D₃. Amaterial of the inner spacers 340-344 can be formed from the same ordifferent material as the gate spacer 322 a-b (e.g., silicon nitride).For example, the inner spacers 340-344 can be formed of silicon nitride,silicoboron carbonitride, silicon carbonitride, silicon carbonoxynitride, or any other type of dielectric material (e.g., a dielectricmaterial having a dielectric constant k of less than about 5)appropriate to the role of forming an insulating gate sidewall spacersof FET devices.

Corresponding to operation 216, FIG. 3-8 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes drain feature 346, source feature 348, drain feature 350,and source feature 352, at one of the various stages of fabrication. Thedrain/source features 346-352 may be formed after the blocking mask 334(shown in FIG. 3-7) is removed. In some embodiments, the drain feature346 may be formed using an epitaxial layer growth process on the exposedends of the etched Si nanostructures 326 a, 328 a, and 330 a on theleft-hand side of the alternating-nanostructure column 324 a. The source348 may be formed using an epitaxial layer growth process on the exposedends of the etched Si nanostructures 326 a, 328 a, and 330 a on theright-hand side of the alternating-nanostructure column 324 a. The drain350 may be formed using an epitaxial layer growth process on the exposedends of the etched Si nanostructures 326 b, 328 b, and 330 b on theleft-hand side of the alternating-nanostructure column 324 b. The source352 is formed using an epitaxial layer growth process on the exposedends of the etched Si nanostructures 326 b, 328 b, and 330 b on theright-hand side of the alternating-nanostructure column 324 b. In someembodiments, the source 348 and drain 350 may be merged with each otherto form a continuous feature or region, as shown in FIG. 3-8.

According to some embodiments, the drain feature 346 and source feature348 are electrically coupled to the Si nanostructures 326 a, 328 a, and330 a; and the drain feature 350 and source feature 352 are electricallycoupled to the Si nanostructures 326 b, 328 b, and 330 b. The Sinanostructures 326 a, 328 a, and 330 a may collectively constitute theconduction channel of a first transistor 354 a; and the Sinanostructures 326 b, 328 b, and 330 b may collectively constitute theconduction channel of a second transistor 354 b. In an example where thememory device is an anti-fuse cell, the first transistor 354 a mayfunction as a programming transistor, and the second transistor 354 bmay function as a reading transistor, electrically connected to theprogramming transistor 354 a in series.

In-situ doping (ISD) may be applied to form the doped drain/sourcefeatures 346-352, thereby creating the necessary junctions for theprogramming transistor 354 a and reading transistor 354 b. N-type andp-type FETs are formed by implanting different types of dopants toselected regions (e.g., drain/source features 346-352) of the device toform the necessary junction(s). N-type devices can be formed byimplanting arsenic (As) or phosphorous (P), and p-type devices can beformed by implanting boron (B).

Corresponding to operation 218, FIG. 3-9 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes an inter-layer dielectric (ILD) material 356, at one ofthe various stages of fabrication. The ILD material 356 can be formed bydepositing an oxide material in bulk (e.g., silicon dioxide) andpolishing the bulk oxide back (e.g., using CMP) to the level of the gatespacers 322 a-b and the hard masks 320 a-b.

Corresponding to operation 220, FIG. 3-10 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,in which the dummy gate stacks 316 a-b (FIG. 3-9) are removed, at one ofthe various stages of fabrication. Subsequently to forming theprotective ILD material 356, the dummy gate stacks 316 a (including thedummy gate 318 a and hard mask 320 a) and 316 b (including the dummygate 318 b and hard mask 320 b), shown in FIG. 3-9, are removed. Thedummy gate stacks 316 a-b can be removed by a known etching process,e.g., RIE or chemical oxide removal (COR).

After the removal of the dummy gate stacks 316 a-b, respective topboundaries of the alternating-nanostructure columns 324 a and 324 b maybe again exposed. Specifically, respective top boundaries of the etchedSi nanostructures 330 a of the alternating-nanostructure column 324 aand the etched Si nanostructures 330 b of the alternating-nanostructurecolumn 324 b may be exposed. Although not shown in the cross-sectionalview of FIG. 3-10, it is appreciated that in addition to the topboundaries, the respective sidewalls of the alternating-nanostructurecolumns 324 a and 324 b, along the Y direction, may also be exposed.

Corresponding to operation 222, FIG. 3-11 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,in which the SiGe sacrificial nanostructures 325 a, 327 a, and 329 a ofthe alternating-nanostructure column 324 a and SiGe sacrificialnanostructures 335 b, 337 b, and 339 b of the alternating-nanostructurecolumn 324 b (shown in FIG. 3-10) are removed, at one of the variousstages of fabrication. The SiGe sacrificial nanostructures 325 a, 327 a,329 a, 335 b, 337 b, and 339 b can be removed by applying a selectiveetch (e.g., a hydrochloric acid (HCl)).

After the removal of the SiGe sacrificial nanostructures 325 a, 327 a,329 a, 335 b, 337 b, and 339 b , respective bottom boundaries of theetched Si nanostructures 326 a, 328 a, and 330 a of thealternating-nanostructure column 324 a and the etched Si nanostructures326 b, 328 b, and 330 b of the alternating-nanostructure column 324 bmay be exposed. As mentioned above, the etched Si nanostructures 326 a,328 a, and 330 a of the alternating-nanostructure column 324 a may becollectively configured as a conduction channel of the programmingtransistor 354 a; and the etched Si nanostructures 326 b, 328 b, and 330b of the alternating-nanostructure column 324 b may be collectivelyconfigured as a conduction channel of the reading transistor 354 b. Assuch, the etched Si nanostructures 326 a, 328 a, and 330 a may herein bereferred to as “conduction channel 360 a;” and the etched Sinanostructures 326 b, 328 b, and 330 b may herein be referred to as“conduction channel 360 b.”

The conduction channels 360 a and 360 b are configured to conductcurrent flowing through the programming transistor 354 a and readingtransistor 354 b, respectively. In general, such a conduction channelhas a length and a width. The length may be in parallel with thecurrent, and the width may be perpendicular to the current. As shown inFIG. 3-11, the conduction channel 360 a may be characterized with alength of about D₁, and the conduction channel 360 b may becharacterized with a length of about D₂. Although three Sinanostructures are formed as the conduction channels of the programmingtransistor 354 a and reading transistor 354 b of the memory device 300,it is understood that a memory device, fabricated by the methoddisclosed herein, can include any number of nanostructures to form itsconduction channel(s) while remaining within the scope of the presentdisclosure.

Corresponding to operation 224, FIG. 3-12 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes gate dielectrics 364 a and 364 b, at one of the variousstages of fabrication. As shown, the gate dielectric 364 a can wraparound each of the Si nanostructures of the conduction channel 360 a(the Si nanostructures 326 a, 328 a, and 330 a); and the gate dielectric364 b can wrap around each of the Si nanostructures of the conductionchannel 360 b (the Si nanostructures 326 a, 328 b, and 330 b). The gatedielectrics 364 a and 364 b include different high-k dielectricmaterials or an identical high-k dielectric material. The gatedielectrics 364 a and 364 b may include a stack of multiple high-kdielectric materials. The gate dielectrics 364 a and 364 b can bedeposited using any suitable method, including, for example, atomiclayer deposition (ALD). In some embodiments, the gate dielectrics 364 aand 364 b may optionally include a substantially thin oxide (e.g.,SiO_(x)) layer. In some embodiments, the gate dielectric 364 a-b may beformed as substantially conformal layers characterized with a thicknessD₅ and a thickness D₆, respectively.

Corresponding to operation 226, FIG. 3-13 is a cross-sectional view ofthe memory device 300, cut by a plane perpendicular to the Y direction,that includes gate metals 366 a and 366 b, at one of the various stagesof fabrication. In some embodiments, the gate 366 a can wrap around eachof the Si nanostructures of the conduction channel 360 a with the gatedielectric 364 a disposed therebetween; and the gate 366 b can wraparound each of the Si nanostructures of the conduction channel 360 bwith the gate dielectric 364 b disposed therebetween. The gate metals366 a-b may be formed of different metal materials or an identical metalmaterial. The gate metals 366 a-b may each include a stack of multiplemetal materials. The gate metals 366 a-b can be deposited using anysuitable method, including, for example, CVD.

Although the gate metals 366 a-b are each shown as a two-dimensionalstructure in FIG. 3-13, it is appreciated that the gate metals 366 a-bare each formed as a three-dimensional structure. Specifically, the gatemetals 366 a-b can each include a number of gate metal sections spacedapart from each other along the Z direction. Each of the gate metalsections can extend not only along a horizontal plane (e.g., the planeexpanded by the X direction and the Y direction), but also along avertical direction (e.g., the Z direction). As such, two adjacent onesof the gate metal sections can adjoin each other so as to wrap around acorresponding Si nanostructure, with a gate dielectric disposedtherebetween.

For example in FIG. 3-13, the gate metal 366 a can include gate metalsections 366 a 1, 366 a 2, 366 a 3, and 366 a 4. The gate metal sections366 a 1 and 366 a 2 may adjoin each other to wrap around the Sinanostructure 330 a, with a portion of the gate dielectric 364 adisposed therebetween. The gate metal sections 366 a 2 and 366 a 3 mayadjoin each other to wrap around the Si nanostructure 328 a, with aportion of the gate dielectric 364 a disposed therebetween. The gatemetal sections 366 a 3 and 366 a 4 may adjoin each other to wrap aroundthe Si nanostructure 326 a, with a portion of the gate dielectric 364 adisposed therebetween. Similarly, the gate metal 366 b can include gatemetal sections 366 b 1, 366 b 2, 366 b 3, and 366 b 4. The gate metalsections 366 b 1 and 366 b 2 may adjoin each other to wrap around the Sinanostructure 330 b, with a portion of the gate dielectric 364 bdisposed therebetween. The gate metal sections 366 b 2 and 366 b 3 mayadjoin each other to wrap around the Si nanostructure 328 b, with aportion of the gate dielectric 364 b disposed therebetween. The gatemetal sections 366 b 3 and 366 b 4 may adjoin each other to wrap aroundthe Si nanostructure 326 b, with a portion of the gate dielectric 364 bdisposed therebetween. In some embodiments, such a gate metal section,together with the corresponding portion of the gate dielectric, that atleast partially wrap around one of the Si nanostructures may becollectively referred to as a gate stack. The gate stack is operativelyassociated with the wrapped Si nanostructure (e.g., modulating thecurrent conducting in the Si nanostructure). The gate stack maysometimes be referred to as an all-around gate stack.

In some embodiments, a number of gate stacks, constituted by thesections of the gate metal 366 a and gate dielectric 364 a, may functionas a gate feature of the programming transistor 354 a to modulate thecurrent conducting from the drain feature 346, through the conductionchannel 360 a, and to the source feature 348; and a number of gatestacks, constituted by the sections of the gate metal 366 b and gatedielectric 364 b, may function as a gate feature of the readingtransistor 354 b to modulate the current conducting from the drainfeature 350, through the conduction channel 360 b, and to the sourcefeature 352.

In the illustrated embodiments of FIGS. 3-1 to 3-13, no inner spacersare formed in the alternating-nanostructure columns 324 a, the gatestacks of the programming transistor 354 a may be in direct contact withthe respective drain and source features, 346 and 348. As such, each ofthe Si nanostructures (e.g., 326 a, 328 a, 330 a) of the conductionchannel 360 a may extend along the X direction by a distance (or length)substantially equal to a distance (or length) by which each of the gatestacks extends along the X direction (e.g., about D₁). The distance bywhich the conduction channel 360 a extends along the X direction maysometimes be referred to as a channel length of the programmingtransistor 354 a. Specifically, each of the gate metal sections 366 a2-4 may be electrically coupled to the drain/source features 346 and 348with the gate dielectric 364 a disposed therebetween. As such, adistance by which each of the gate metal sections 366 a 2-4 extendsalong the X direction is about D₁−2×D₅.

On the other hand, the inner spacers 340-344 are formed in thealternating-nanostructure columns 324 b, the gate stacks of the readingtransistor 354 b may be electrically isolated from the respective drainand source features, 350 and 352 by the inner spacers 340-344. As such,each of the Si nanostructures (e.g., 326 b, 328 b, 330 b) of theconduction channel 360 b may extend along the X direction by a distance(or length), e.g., about D₁, substantially greater than a distance (orlength) by which each of the gate stacks extends along the X direction(e.g., about D₂−2×D₄). The distance by which the conduction channel 360b extends along the X direction may sometimes be referred to as achannel length of the reading transistor 354 b. Specifically, each ofthe gate metal sections 366 b 2-4 may be electrically isolated from thedrain/source features 350 and 352 with the gate dielectric 364 b and therespective one of the inner spacers 340-344 disposed therebetween. Assuch, a distance by which each of the gate metal sections 366 b 2-4extends along the X direction is about D₂−2×D₄−2×D₆.

By forming the programming transistor and reading transistor of a memorycell in such a configuration, the contact area of each of the gatestacks to the source/drain features for the programming transistor canbe increased, which may advantageously reduce VBD and TBD of theprogramming transistor. Concurrently, keeping the inner spacers for thereading transistor, the parasitic capacitances can be effectivelysuppressed so as not to compromise switching speed of the readingtransistor.

After forming the gate metals 366 a-b, one or more interconnectionstructures may be formed to connect each of the gate metal 366 a, thegate metal 366 b, and the source feature 352 to connect the memorydevice 300 to other components or devices. For example, one or moreinterconnection structures (e.g., a via structure typically known as VG)may be formed over the gate metal 366 a to connect it to one or moreupper metal layers, which may include a programming word line (WLP); oneor more interconnection structures (e.g., VG) may be formed over thegate metal 366 b to connect it to one or more upper metal layers, whichmay include a reading word line (WLR); and one or more interconnectionstructures (e.g., a metal structure typically known as MD, a viastructure typically known as VD)) may be formed in or over the ILD 356and over the source feature 352 to connect it to one or more upper metallayers, which may include a bit line (BL). As such, the memory device300, as an example anti-fuse memory cell, can be connected to one ormore other memory cells similar to the memory device 300. For example, anumber of such memory device 300 may be arranged (e.g., coupled) to eachother by respective WLPs, reading WLs, and BLs to form a memory array.

FIG. 4 illustrates a cross-sectional view of another example memorydevice 400, in accordance with some embodiments. The memory device 400may be substantially similar to the memory device 300 (FIGS. 3-1 to3-13) except that both of the programming transistor and readingtransistor of the memory device 400 include inner spacers. Thus, thefollowing discussions shall be focused on the difference between thememory devices 300 and 400.

As shown, the memory device 400 includes a programming transistor 404 aand reading transistor 404 b formed on a substrate 402. Similar to theprogramming transistor 354 a, the programming transistor 404 a alsoincludes a gate metal 406 a, a gate dielectric 408 a, gate spacers 409a, a number of Si nanostructures collectively functioning as aconduction channel 410 a, a drain feature 412, and a source feature 414.Similar to the reading transistor 354 b, the reading transistor 404 balso includes a gate metal 406 b, a gate dielectric 408 b, gate spacers409 b, a number of Si nanostructures collectively functioning as aconduction channel 410 b, a drain feature 416, and a source feature 418.At least a portion of each of the programming transistor 404 a andreading transistor 404 b is embedded in an ILD 420.

Different from the memory device 300, both of the programming transistor404 a and reading transistor 404 b include inner spacers. Specifically,the gate metal 406 a of the programming transistor 404 a includes gatemetal sections 406 a 1, 406 a 2, 406 a 3, and 406 a 4. The gate metalsection 406 a 1 and a portion of the gate dielectric 408 a mayconstitute a first one of a number of gate stacks for the programmingtransistor 404 a; the gate metal section 406 a 2 and a portion of thegate dielectric 408 a may constitute a second one of the gate stacks forthe programming transistor 404 a; the gate metal section 406 a 3 and aportion of the gate dielectric 408 a may constitute a third one of thegate stacks for the programming transistor 404 a; and the gate metalsection 406 a 4 and a portion of the gate dielectric 408 a mayconstitute a fourth one of the gate stacks for the programmingtransistor 404 a. Each of the gate stacks can at least partially wraparound a corresponding Si nanostructure of the conduction channel 410 a.

Similarly, the gate metal 406 b of the reading transistor 404 b includesgate metal sections 406 b 1, 406 b 2, 406 b 3, and 406 b 4. The gatemetal section 406 b 1 and a portion of the gate dielectric 408 b mayconstitute a first one of a number of gate stacks for the readingtransistor 404 b; the gate metal section 406 b 2 and a portion of thegate dielectric 408 b may constitute a second one of the gate stacks forthe reading transistor 404 b; the gate metal section 406 b 3 and aportion of the gate dielectric 408 b may constitute a third one of thegate stacks for the reading transistor 404 b; and the gate metal section406 b 4 and a portion of the gate dielectric 408 b may constitute afourth one of the gate stacks for the reading transistor 404 b. Each ofthe gate stacks can at least partially wrap around a corresponding Sinanostructure of the conduction channel 410 b.

Some of the gate stacks of the programming transistor 404 a are isolatedfrom the respective drain/source features, 412 and 414, by inner spacers424, for example, the gate stack including the gate metal section 406 a2, the gate stack including the gate metal section 406 a 3, and the gatestack including the gate metal section 406 a 4. Some of the gate stacksof the reading transistor 404 b are isolated from the respectivedrain/source features, 416 and 418, by inner spacers 426, for example,the gate stack including the gate metal section 406 b 2, the gate stackincluding the gate metal section 406 b 3, and the gate stack includingthe gate metal section 406 b 4. In some embodiments, the inner spacers424-426 may be formed of a dielectric material selected from: siliconoxide, silicon nitride, silicon oxynitride, SiBCN, SiOCN, SiOC, or acombination thereof.

To the extent of reducing VBD/TBD of the programming transistor 404 awhile suppressing the parasitic capacitances of the reading transistor404 b, the inner spacers 424 and 426 have different effectivecapacitance. In some embodiments a thickness of inner spacers 424 issubstantially equal to that of inner spacers 426, but a dielectricconstant of inner spacers 424 is different from that of inner spacers426. For example, the inner spacers 424 may be formed of a dielectricmaterial characterized with a dielectric constant higher than thedielectric constant of a dielectric material of the inner spacers 426.In another example, the inner spacers 424 and 426 may be formed to havedifferent geometric dimensions. Each of the inner spacers 424 may extendalong the X direction by a distance (sometimes referred to as innerspacers' width), D₇, and along the Z direction by a distance (sometimesreferred to inner spacers' height), D₉; and each of the inner spacers426 may extend along the X direction by a distance (width), D₈, andalong the Z direction by a distance (height), D₁₀. In some embodiments,a sum of the D₇'s of the inner spacers 424 may be selected to be lessthan a sum of the D₈'s of the inner spacers 426; and/or a sum of the D₉sof the inner spacers 424 may be selected to be less than a sum of theD₁₀s of the inner spacers 426. In at least one embodiment, inner spacers424 and inner spacers 426 have different geometric dimensions but have asame dielectric constant.

Given a non-zero thickness of the inner spacers 424 and 426, each of theSi nanostructures of the conduction channels 410 a-b may becharacterized with a channel length greater than a distance by which thecorresponding gate stacks extend along the X direction. For example inFIG. 4, the Si nanostructures of the conduction channel 410 a may have achannel length of D₁₁ greater than a distance by which the correspondinggate stacks extend along the X direction (e.g., D₁₁−2×D₇). In anotherexample, the Si nanostructures of the conduction channel 410 b may havea channel length of D₁₂ greater than a distance by which thecorresponding gate stacks extend along the X direction (e.g., D₁₂−2×D₈).

To make the memory device 400, a method substantially similar to themethod 200 of FIG. 2 may be used. For example, operation 210 may beskipped. As such, the first alternating-nanostructure column is alsoexposed while removing end portions of the first nanostructures of thefirst alternating-nanostructure column (operation 212). Further,operation 212 may be changed to remove end portions of the firstnanostructures of both of the first and second alternating-nanostructurecolumns.

FIGS. 5, 6, and 7 illustrate various example nanostructure transistorsincluding partial inner spacers, in accordance with some embodiments.The term “partial inner spacers” refer to the inner spacers of ananostructure transistor that do not completely isolate thecorresponding gate stacks from respective source or drain features. Incontrast, “complete inner spacers” may refer to the inner spacers of ananostructure transistor that completely isolate the corresponding gatestacks from respective source or drain features. In some embodiments,the nanostructure transistor with partial inner spacers are not limitedto being configured as the programming transistor or the readingtransistor of an anti-fuse cell. However, to the extent of reducingVBD/TBD of the programming transistor while suppressing the parasiticcapacitances of the reading transistor, the programming transistor andreading transistor, for example, may be configured as a nanostructuretransistor with partial inner spacers and a nanostructure transistorwith complete inner spacers, respectively. In another example, theprogramming transistor and reading transistor may be configured as ananostructure transistor with partial inner spacers and a nanostructuretransistor also with partial inner spacers, respectively, but a sum ofthe widths (or heights) of the partial inner spacers in the programmingtransistor is less than a sum of the widths (or heights) of the partialinner spacers in the reading transistor.

Referring to FIG. 5, a nanostructure transistor 500, including partialinner spacers, is depicted. The nanostructure transistor 500 is formedon a substrate 502. The nanostructure transistor 500 includes a gatemetal 506, a gate dielectric 508, gate spacers 510, a number of Sinanostructures collectively functioning as a conduction channel 512, adrain feature 514, and a source feature 516. At least a portion of thenanostructure transistor 500 is embedded in an ILD 518.

The gate metal 506 includes gate metal sections 506 a, 506 b, 506 c, and506 d. The gate metal section 506 a and a portion of the gate dielectric508 may constitute a first one of a number of gate stacks for thenanostructure transistor 500; the gate metal section 506 b and a portionof the gate dielectric 508 may constitute a second one of the gatestacks for the nanostructure transistor 500; the gate metal section 506c and a portion of the gate dielectric 508 may constitute a third one ofthe gate stacks for the nanostructure transistor 500; and the gate metalsection 506 d and a portion of the gate dielectric 508 may constitute afourth one of the gate stacks for the nanostructure transistor 500. Eachof the gate stacks can at least partially wrap around a corresponding Sinanostructure of the conduction channel 512. In the illustratedembodiment of FIG. 5, the nanostructure transistor 500 include partialinner spacers 520 that only isolate the corresponding gate stacks fromthe source feature 516, instead of both the drain feature 514 and sourcefeature 516.

Referring to FIG. 6, another nanostructure transistor 600, includingpartial inner spacers, is depicted. The nanostructure transistor 600 isformed on a substrate 602. The nanostructure transistor 600 includes agate metal 606, a gate dielectric 608, gate spacers 610, a number of Sinanostructures collectively functioning as a conduction channel 612, adrain feature 614, and a source feature 616. At least a portion of thenanostructure transistor 600 is embedded in an ILD 618.

The gate metal 606 includes gate metal sections 606 a, 606 b, 606 c, and606 d. The gate metal section 606 a and a portion of the gate dielectric608 may constitute a first one of a number of gate stacks for thenanostructure transistor 600; the gate metal section 606 b and a portionof the gate dielectric 608 may constitute a second one of the gatestacks for the nanostructure transistor 600; the gate metal section 606c and a portion of the gate dielectric 608 may constitute a third one ofthe gate stacks for the nanostructure transistor 600; and the gate metalsection 606 d and a portion of the gate dielectric 608 may constitute afourth one of the gate stacks for the nanostructure transistor 600. Eachof the gate stacks can at least partially wrap around a corresponding Sinanostructure of the conduction channel 612. In the illustratedembodiment of FIG. 6, the nanostructure transistor 600 include partialinner spacers 620 that only isolate the corresponding gate stacks fromthe drain feature 614, instead of both the drain feature 614 and sourcefeature 616.

Referring to FIG. 7, yet another nanostructure transistor 700, includingpartial inner spacers, is depicted. The nanostructure transistor 700 isformed on a substrate 702. The nanostructure transistor 700 includes agate metal 706, a gate dielectric 708, gate spacers 710, a number of Sinanostructures collectively functioning as a conduction channel 712, adrain feature 714, and a source feature 716. At least a portion of thenanostructure transistor 700 is embedded in an ILD 718.

The gate metal 706 includes gate metal sections 706 a, 706 b, 706 c, and706 d. The gate metal section 706 a and a portion of the gate dielectric708 may constitute a first one of a number of gate stacks for thenanostructure transistor 700; the gate metal section 706 b and a portionof the gate dielectric 708 may constitute a second one of the gatestacks for the nanostructure transistor 700; the gate metal section 706c and a portion of the gate dielectric 708 may constitute a third one ofthe gate stacks for the nanostructure transistor 700; and the gate metalsection 706 d and a portion of the gate dielectric 708 may constitute afourth one of the gate stacks for the nanostructure transistor 700. Eachof the gate stacks can at least partially wrap around a corresponding Sinanostructure of the conduction channel 712. In the illustratedembodiment of FIG. 7, the nanostructure transistor 700 include partialinner spacer 720 that only isolates the corresponding gate stack fromthe source feature 716 (i.e., leaving the other side of thecorresponding gate stack in direct contact with the drain feature 714),inner spacer 724 that only isolates the corresponding gate stack fromthe drain feature 714 (i.e., leaving the other side of the correspondinggate stack in direct contact with the source feature 716), and innerspacer 726 that only isolates the corresponding gate stack from thesource feature 716 (i.e., leaving the other side of the correspondinggate stack in direct contact with the drain feature 714).

In one aspect of the present disclosure, a memory device is disclosed.The memory device includes a plurality of first nanostructures stackedon top of one another; a plurality of first gate stacks where twoadjacent ones of the first gate stacks wrap around a corresponding oneof the plurality of first nanostructures; a plurality of secondnanostructures stacked on top of one another; a plurality of second gatestacks where two adjacent ones of the second gate stacks wrap around acorresponding one of the plurality of second nanostructures; a firstdrain/source feature electrically coupled to a first end of the firstnanostructures; a second drain/source feature electrically coupled toboth of a second end of the first nanostructures and a first end of thesecond nanostructures; and a third drain/source feature electricallycoupled to a second end of the second nanostructures. At least one ofthe plurality of first gate stacks is in direct contact with at leastone of the first drain/source feature or the second drain/sourcefeature.

In another aspect of the present disclosure, a memory cell is disclosed.The memory cell includes a first transistor, a second transistorelectrically coupled to the first transistor in series. The firsttransistor includes a plurality of first nanosheets spaced apart fromone another along a vertical direction, where the plurality of firstnanosheets have a first length along a horizontal direction; and aplurality of first all-around gate stacks operatively associated withthe plurality of first nanosheets, where the plurality of firstall-around gate stacks have a second length along the horizontaldirection, the second length is either equal to or less than the firstlength. The second transistor includes a plurality of second nanosheetsvertically spaced apart from one another, where the plurality of secondnanosheets have a third length along the horizontal direction; and aplurality of second all-around gate stacks operatively associated withthe plurality of second nanosheets, where the plurality of secondall-around gate stacks have a fourth length along the horizontaldirection, the fourth length is less than the third length.

In yet another aspect of the present disclosure, a method forfabricating a memory device is disclosed. The method includes forming afirst stack over a substrate. The first stack includes a firstnanosheet, a second nanosheet over the first nanosheet, and a thirdnanosheet over the second nanosheet. The method includes forming asecond stack over the substrate. The second stack includes a fourthnanosheet, a fifth nanosheet over the fourth nanosheet, and a sixthnanosheet over the fifth nanosheet. The method includes removingrespective end portions of the fourth nanosheet and the sixth nanosheetwhile covering the first stack. The method includes forming a pluralityof spacers at the respective etched end portions of the fourth nanosheetand the sixth nanosheet while still covering the first stack.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for fabricating a memory device,comprising: forming a first stack over a substrate, the first stackcomprising a first nanosheet, a second nanosheet over the firstnanosheet, and a third nanosheet over the second nanosheet; forming asecond stack over the substrate, the second stack comprising a fourthnanosheet, a fifth nanosheet over the fourth nanosheet, and a sixthnanosheet over the fifth nanosheet; removing respective end portions ofthe fourth nanosheet and the sixth nanosheet while covering the firststack; and forming a plurality of spacers at the respective etched endportions of the fourth nanosheet and the sixth nanosheet while stillcovering the first stack.
 2. The method of claim 1, subsequently toforming the spacers, further comprising: forming a first source/drainfeature in physical contact with a first end of each of the firstnanosheet, the second nanosheet, and the third nanosheet; forming asecond source/drain feature in physical contact with a second end ofeach of the first nanosheet, the second nanosheet, and the thirdnanosheet and in physical contact with a first end of the fifthnanosheet; and forming a third source/drain feature in physical contactwith a second end of the fifth nanosheet.
 3. The method of claim 2,wherein the first source/drain feature is epitaxially grown from thesecond nanosheet, and the third source/drain feature is epitaxiallygrown from the fifth nanosheet.
 4. The method of claim 2, wherein thesecond source/drain feature includes a first portion epitaxially grownfrom the second nanosheet and a second portion epitaxially grown fromthe fifth nanosheet.
 5. The method of claim 2, further comprising:replacing the first nanosheet and the third nanosheet with a first gatestack to wrap around the second nanosheet; and replacing the fourthnanosheet and the sixth nanosheet with a second gate stack to wraparound the fifth nanosheet.
 6. The method of claim 5, wherein the firstgate stack is in direct contact with the first source/drain feature andwith the second source/drain feature.
 7. The method of claim 5, whereinthe second gate stack is electrically isolated from the secondsource/drain feature and the third source/drain feature by the pluralityof spacers.
 8. The method of claim 2, wherein the first to thirdsource/drain features have a same conductive type.
 9. The method ofclaim 1, wherein the first to sixth nanosheets extend along a samehorizontal direction.
 10. A method for fabricating a memory device,comprising: forming a first transistor including: a plurality of firstnanostructures extending along a lateral direction; a plurality of firstgate stacks, two adjacent ones of the first gate stacks wrapping arounda corresponding one of the plurality of first nanostructures; a firstepitaxially grown feature in physical contact with a first end of eachof the first nanostructures and to a first end of each of the first gatestacks; and a second epitaxially grown feature in physical contact witha second end of each of the first nanostructures and with a second endof each of the first gate stacks; and forming a second transistoradjacent the first transistor along the lateral direction and including:a plurality of second nanostructures extending along the lateraldirection; a plurality of second gate stacks, two adjacent ones of thesecond gate stacks wrapping around a corresponding one of the pluralityof second nanostructures; the second epitaxially grown feature which isfurther in physical contact with a first end of each of the secondnanostructures; and a third epitaxially grown feature in physicalcontact with a second end of each of the second nanostructures.
 11. Themethod of claim 10, wherein each of the plurality of second gate stacksis electrically isolated from the second epitaxially grown feature by afirst dielectric spacer, and electrically isolated from the thirdepitaxially grown feature by a second dielectric spacer.
 12. The methodof claim 11, further comprising covering at least the firstnanostructures of the first transistor, while forming the first andsecond dielectric spacers.
 13. The method of claim 10, wherein each ofthe plurality of first gate stacks includes a first gate metal and afirst gate dielectric, and each of the plurality of second gate stacksincludes a second gate metal and a second gate dielectric.
 14. Themethod of claim 13, wherein the first gate dielectric is configured tobe broken down by applying a voltage on the first gate metal therebyelectrically connecting the first gate stacks to the second epitaxiallygrown feature.
 15. The method of claim 10, wherein the first transistoroperatively functions as a programming transistor of an anti-fuse memorycell, and the second transistor operatively functions as a readingtransistor of the anti-fuse memory cell.
 16. The method of claim 10,wherein the first to third epitaxially grown features have a sameconductive type.
 17. A method for fabricating a semiconductor device,comprising: forming, over a substrate, a first stack including aplurality of first nanostructures and a plurality of secondnanostructures, wherein the plurality of first nanostructures and theplurality of second nanostructures are alternatingly arranged on top ofone another; forming, over the substrate, a second stack including aplurality of third nanostructures and a plurality of fourthnanostructures, wherein the plurality of third nanostructures and theplurality of fourth nanostructures are alternatingly arranged on top ofone another; removing respective end portions of each of the pluralityof third nanostructures while covering the first stack; forming aplurality of spacers at the respective etched end portions of the thirdnanostructures while still covering the first stack; forming a firstsource/drain feature in physical contact with a first end of each of thefirst nanostructures and the second nanostructures; forming a secondsource/drain feature in physical contact with a second end of each ofthe first nanostructures and the second nanostructures, and in physicalcontact with a first end of each of the fourth nanostructures; andforming a third source/drain feature in physical contact with a secondend of each of the fourth nanostructures.
 18. The method of claim 17,further comprising: removing the plurality of first nanostructures;forming a plurality of first gate stacks, two adjacent ones of the firstgate stacks wrapping around a corresponding one of the plurality ofsecond nanostructures; removing respective remaining portions of theplurality of third nanostructures; and forming a plurality of secondgate stacks, two adjacent ones of the second gate stacks wrapping arounda corresponding one of the plurality of fourth nanostructures.
 19. Themethod of claim 17, wherein the first source/drain feature and secondsource/drain feature are in direct contact with ends of each of thefirst gate stacks, respectively.
 20. The method of claim 17, wherein thefirst to third source/drain features have a same conductive type.